Hyaloclastite

Composition

Several minerals are found in hyaloklastischen rocks. Sideromelane is an olive, obsidian- like, basaltic glass, which was rapidly quenched in water. It is transparent and pure, it lacks in contrast to the more common Tachylit the interspersed iron oxide crystals.

The glass fragments are usually surrounded by a yellow or brown layer of palagonite, which is formed in the reaction of sideromelane with water.

Formation

Hyaloclastite arise during cooling of lava under exclusion of air and is therefore a characteristic mineral in the area of subglacial or submarine volcanoes and Tuya. The fragmentation of volcanic glass occurs by the force of a volcanic explosion or by a thermal shock during rapid cooling.